The present invention is concerned with a device for smoothening the unwinding of the loops of yarn forming the weft reserve on the cylinder of a weft supplier for a fluid jet looms, such as an air jet loom or a water jet loom.
As well known, the weft suppliers are capable of storing a yarn reserve, in form of loops wound on a stationary cylinder, and of supplying the loom by unwinding a number of loops matching the length L of yarn required by the loom at each weft insertion, the length L equalling the width of the fabric being woven.
In the specific case of fluid jet looms, the weft supplier also has the task of pre-measuring the length L and such a task is performed by counting, typically by means of a photo-sensor, the loops of yarn which have been unwound; being
L=nxcfx80D
where n is the number of loops which have been unwound and D is the diameter of the stationary cylinder of the supplier.
The unwinding of the yarn is controlled by an electrically-controlled stop comprising a movable finger that, when energized, engages the cylinder, in order to block the running of the yarn when the n-th loop has been reached.
The operating time of the stop device mounted on current weft suppliers is quite short, typically in the range of 10 to 20 milliseconds, due to the high operating speed of a modern fluid jet loom, which is able to insert about 1500 meters of weft per minute. This fact causes a peak in the mechanical tension of the yarn, downstream of the stop finger, which tension peak must be smoothed in order to prevent breaking of the yarn. Upstream of the stop finger, it causes slackening of one or more of the loops that are wound on the terminal portion of the cylinder, because of the inertia dragging actions caused by the quick stopping of the running yarn.
Nowadays, in order to avoid such phenomena and to smoothen the unwinding and stopping of the yarn as well as possible, smoothening devices of several kinds are used, typically providing a modulated braking of the weft yarn, e.g. as disclosed in prior European Patent Application No. 99112784.6. Such devices, which are interposed between the weft supplier and the loom, are put into action during the unwinding of the last loop, in order to slow down the running of the yarn while maintaining it in tension, in order to attenuate the deceleration peak of the yarn when the stop finger is driven.
Although such known smootheners operate satisfactorily to damp the mechanical tension in the yarn downstream of the stop finger, they are not capable of effectively preventing the slackening of the loops upstream of the stop finger. Moreover, such capability depends on the characteristics of the yarn being processed. Particularly with yarns having a slippery surface and/or having poor elasticity, the above known modulated-braking devices are unable to prevent slackening of one or more of the loops wound on the terminal portion of the cylinder of the supplier, upstream of the stop finger, probably because of their limited ability to brake the yarn and to control its mechanical tension.
Such a circumstance is a serious drawback hindering the regular supply of the weft yarn because, at the time of releasing the next weft length, when the weft yarn is released by the stop finger for restoring its running, two or more loops, rather than single loops, are unwound from the cylinder at the same time, thereby causing the weft length L to be correspondingly increased by a factor n+x (rather than n), and consequently causing the loom to stop, due to the so-called xe2x80x9clong weftxe2x80x9d signal generated by the optical sensors checking the insertion.
The present invention has the main object of removing the above drawback, and achieves this and other objects, such as will become apparent from the following disclosure, by means of a device for smoothening the unwinding of the weft yarn having the features defined in the attached claims.
According to the invention, a pneumatic device subjects the weft yarn to a fluid flow, such as an air stream flowing in the same direction as the yarn, in the path between the supplier and the loom. The flow, independently of the size and surface characteristics of the yarn and of the efficiency of the modulated braking device mounted in the same path, maintains the yarn under a constant mechanical tension, whereby, surprisingly for persons skilled in the art, the slackening of the loops wound on the terminal portion of the cylinder of the supplier, upstream of said stop finger, is prevented.
According to an advantageous embodiment of the invention, the pneumatic device that generates the fluid flow for the mechanical tensioning of the yarn comprises the already existing nozzle, mounted on a terminal yarn-guide eyelet and coupled to the modulated braking device, in which pressurized air is supplied for the pneumatic threading of the braking device. The rate of the flow is controlled by at least one flow controller which is operatedxe2x80x94according to the features of the weft yarn and to the parameters of the weaving processxe2x80x94so that the mechanical tensioning of the yarn will prevent the slackening of the terminal loops on the cylinder of the supplier.